Extreme Ultraviolet Lithography (EUVL) is an emerging lithography technology for the 22 nm node and beyond for MPU/DRAMs [MPU—Micro Processing Unit & DRAM—Dynamic Random Access Memory). The scanners needed to support EUV lithography have been developed and are currently in use on a limited scale to demonstrate the potential of this new technology. Key components of EUVL scanners include reflective optics for directing and controlling exposure light from an EUV source to enable patterning of features on silicon wafers. The reflecting optic elements typically include a substrate with a series of coating layers. Due to the short wavelength of EUV exposure light (e.g. 13.5 nm) and the high powers needed for high throughput processing of wafers, significant heating of the reflecting optic elements can occur. In order to avoid distortions in the patterns transferred to the wafers, it is necessary for the reflecting optic elements to maintain constant performance over a wide range of temperature. The requirement for temperature-stable performance has motivated the development of low thermal expansion materials for use as substrate materials for optics in EUVL scanners.
Initially, glass-ceramic materials were considered as substrate materials for EUV optics. Glass-ceramic materials, however, proved to be inadequate and have been replaced by low expansion titania-silica glass (such as ULE® glass from Corning Incorporated). The advantages of ULE® titania-silica glass include polishability to an extremely flat finish, low coefficient of thermal expansion (CTE), and dimensional stability over a range of temperatures. The roadmap for semiconductor technology calls for ever decreasing feature sizes and ever increasing demands on lithography systems. In order to meet the challenges needed to advance the capabilities of lithography systems, it is necessary to develop new materials for scanner optics that are increasingly insensitive to thermal effects.